1. Field of the Invention
The present invention relates to an optical beam splitter used in recording halftone plate duplicate images.
2. Prior Art
There exists an optical beam splitter which is used in recording halftone plate duplicate images on a recording material by controlling a light-exposure device in accordance with image signals obtained by photoelectric manipulation of an original image so that a plurality of beams are independently modulated by the image signals.
In the conventional method, a multiple number of light beams, which are lined up in a row, are independently modulated by image signals so that they are relatively scanned on the surface of recording materials. In most cases, the multiple number of light beams are obtained by reflecting a single light beam, which comes from an argon laser, on a plurality of fully reflective and semi-reflective mirrors so that the beam is split by these mirrors. The light beams thus obtained are independently modulated by a modulator, reduced in diameter by a crystal optical system and then directed onto the surface of the recording material.
It is also a prior art practice for a single light beam to be split into a multiple number of light beams lined up in a row by repeating internal reflections of the light beam using a single optical beam splitter. This method is disclosed, for example, in Japanese Patent Application Laid-Open Nos. 52-122135 and 58-10713. In this art, one surface of a single glass plate is coated with a fully reflective film, while the other surface of the glass plate is coated in different regions with semi-reflective films having different reflectivities.
These semi-reflective film coatings are described in greater detail as follows:
When a single light beam is to be split into ten (10) light beams, for example, the first region is coated with a semi-reflective film having a reflectivity of 9/10 (i.e., a transmissibility of 1/10); the next region is coated with a semi-reflective film having a reflectivity of 8/9 (i.e., a transmissibility of 1/9). The next region is coated with a semi-reflective film having a reflectivity of 7/8 (i.e., a transmissibility of 1/8), and the successive regions are coated with semi-reflective films having reflectivities gradually decreasing in a numerical series so that the last region is coated with a semi-reflective film having a reflectivity of 1/2 (i.e., a transmissibility of 1/2).
The techniques in which a single laser beam generated by an argon laser is split into a multiple number of laser beams via mirrors and optical fibers as described above requires a large amount of space for installation of the apparatus. Also, adjustment of the mirrors and optical fibers requires a high degree of skill and considerable time and is therefore very inconvenient.
The above-mentioned spatial drawbacks are eliminated by the optical beam splitters described in Japanese Patent Application Laid Open Nos. 58-10713 and 52-122135. In these optical beam splitters, however, in order for a single light beam to be split into ten (10) light beams, semi-reflective coatings having different reflectivities as described above must be coated in nine extremely narrow regions, i.e. regions with a width of 2 mm or less. Flaw in the coating work, even to one region, is not permissible, and therefore, this coating process is extremely difficult.
The optical beam splitter disclosed in Japanese Patent Application Laid-Open No. 52-122135 is advantageous in that there is no need to increase the size of the crystal optical system even if the single beam is split into a large number of beams. However, since the split light beams are not parallel, the modulation efficiency of the modulator drops, and light leakage may occur. Furthermore, the formation of the acoustic electrodes of the modulator is very difficult. It is also difficult to maintain the thickness and angular position of the two surfaces of this optical beam splitter at prescribed values with an ultrahigh degree of precision. Thus, the manufacture of this optical beam splitter involves great difficulty. If the thickness and angular positions of the two surfaces of the optical beam splitter differ even slightly from the predetermined values, the focal distances of the various light beams become greatly different. Thus, the spacing between the optical beam splitter and modulator, as well as the spacing between the modulator and crystal optical system, will be unavoidably different in each individual apparatus.